1. Field of the Invention
The present invention relates to the field of combined cycle power stations (combined plant). It relates to a process for the recovery of water which arises in a combined plant, and also to a combined cycle power station.
2. Related Art
Combined cycle power stations normally burn natural gas, which produces water vapor during combustion. 1 kg of natural gas burns to about 2 kg of water and also CO2. The flue gas with the water contained therein is usually discharged through a chimney at elevated temperatures, without the water being used to advantage. The same also holds for water or steam, which in other cases is injected or sprayed in during combustion and thus becomes a component of the flue gas. On the other hand, water already forms an increasingly valuable resource at the present time.
The simplified diagram of an exemplary combined cycle power station from the state of the art is reproduced in FIG. 1. The combined cycle power station 10 essentially comprises a gas turbine plant 11, a waste heat boiler (heat recovery steam generator HRSG) 33, and a steam turbine 25, which are connected together. The gas turbine plant 11 consists of a compressor 14 and a turbine 17, which are arranged on a rotor 16, and also a combustion chamber 15. In operation, the compressor 14 sucks in air through an air inlet 12, compresses it, and delivers the compressed air to the combustion chamber 15, where it enters combustion as combustion air, together with a liquid or gaseous fuel 19 (e.g., natural gas) which is fed in through a fuel supply duct 21. In addition, water or steam 36 can be injected or sprayed into the combustion chamber 15 to reduce the combustion temperature. The hot combustion gases are conducted from the combustion chamber 15 into the turbine 17, where they set the rotor 16 in rotation. The flue gas 42 exiting the turbine 17 is then conducted through the waste heat boiler 33 for the production of steam, where it flows in succession through a superheater 40, an evaporator 39, and a preheater (economizer) 34, and gives up heat stepwise. The cooled flue gas 42 finally leaves the waste heat boiler 33 and is usually, possibly after a flue gas cleaning, discharged from a chimney.
The superheater 40, evaporator 39 and preheater 34 are connected in series as part of a water/steam circuit, in which the steam turbine is also connected. The exhaust steam from the steam turbine 25 passes into a condenser 26 and condenses there. The condensate is pumped by a condensate pump 28, possibly with the addition of additional water 27, through a feed water duct 29 to a feed water container 30 with degasser. The degassed condensate is then pumped as feed water 32 through the preheater 34 by a boiler feed pump 31, and as pressurized feed water 35 to a steam drum 37 with the connected evaporator 39. The steam then passes to the superheater 40, where it is superheated, and finally drives the steam turbine 25 as superheated steam 41. Both the steam turbine 25 and also the gas turbine plant 11 respectively drive a generator 13 or 24 which produces electrical current. Means for the recovery of water or water vapor contained in the flue gas are not provided here.
The invention therefore has as an object to provide a process and also a combined cycle power station with which the water contained in the flue gas is recovered and can be advantageously reused.
An aspect of the invention involves configuring the combined cycle power station and conducting the process so that the water is condensed out of the flue gas after leaving the waste heat boiler and is separated in liquid form.
A first preferred embodiment of the process according to the invention is characterized in that the flue gas is expanded for condensing the water out, with output of work. The expansion is preferably carried out by means of a utilization turbine. In particular, the waste heat boiler is operated for this purpose at a pressure exceeding the ambient air pressure by several bar, preferably 2-5 bar, and the flue gas is brought to the ambient air pressure by the subsequent expansion. Alternatively to this, the waste heat boiler can be operated at about the ambient pressure with respect to the flue gas, the flue gas subsequently being expanded into a vacuum, and the flue gas being compressed again to ambient air pressure after the separation of water. In both cases, the water is separated from the flue gas particularly during the expansion or in the utilization turbine itself, and/or in a droplet separator following the expansion or the utilization turbine.
A second preferred embodiment of the process according to the invention is characterized in that the waste heat boiler is operated with respect to the flue gas at a pressure exceeding the ambient air pressure by several bar, preferably 2-5 bar, in that the water is condensed out of the flue gas on cold surfaces after leaving the waste heat boiler, in particular on the cold tubes of a heat exchanger, and in that the dewatered flue gas is brought to the ambient air pressure by a subsequent expansion; here also, the expansion is preferably carried out by means of a utilization turbine.
A preferred embodiment of the combined cycle power station according to the invention is distinguished in that the means for condensation and separation comprise means for the expansion of the flue gas, preferably in the form of a utilization turbine.
A first development of this embodiment is characterized in that the expansion means or the utilization turbine is followed by a droplet separator.
A second development of this embodiment is characterized in that the expansion means comprises a utilization turbine working in vacuum, and that a compressor follows the utilization turbine.
A third development of this embodiment is characterized in that condensation means, particularly in the form of a heat exchanger, is arranged between the waste heat boiler and the expansion means.